1. Field of the Invention
The present invention relates to a polymer-based optical waveguide circuit stable to the changes of particularly, a polarization and ambient temperature which is profitably used as a wavelength division multiplexer or demultiplexer in, for example, fiber optic communication.
2. Description of the Related Art
In a dense wavelength division multiplexing (DWDM) system applied to optical carriers belonging to S, C and L bands, the wavelength independent waveguide coupler has been used that is capable of routing optical carriers comprising 80–100 nm bands around 1550 nm into two channels at a specified split ratio, being unaffected with their wavelengths. The coupler that routes optical carriers at a split ratio of 1:1 is called a 3 dB coupler and has been used in multiple fiber optic communication systems.
Such a wavelength independent waveguide coupler includes a Mach-Zehnder interferometer type optical circuit 17 as shown in FIG. 9. The circuit comprises two waveguide cores 13, 14 formed on the surface of a substrate 20 prepared from a quartz or silicon wafer. (K. Jinguji et al., “Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio,” Electron. Lett., 1990, Vol. 26, No. 17, pp. 1326–1327 and The publication of unexamined application No.213829/1991).
The waveguide cores 13, 14 are covered with lower and upper cladding layers 18, 19 both of which are formed on the substrate 20. When the core and cladding layers are mainly composed of silicon dioxide (SiO2), the resulting optical circuit is called a quartz waveguide. When they are composed of a polymer, the resulting optical circuit is called a polymeric waveguide.
The Mach-Zehnder interferometer type optical circuit 17 comprises two directional couplers 15, 16 which are obtained by bringing the two waveguide cores 13, 14 close to and in parallel with each other. The Mach-Zehnder interferometer type optical circuit 17 receives an optical signal having a band width of 80–100 nm around 1550 nm either from a terminal 13a or 14a connected respectively to the waveguide core 13 or 14 and splits the signal at a split ratio of 1:1 to deliver two outputs each having an intensity half that of the transmitted signal from terminals 13b, 14b, or the other terminals of the waveguide cores 13, 14.
An application of such a Mach-Zender interferometer type optical circuit 17 includes 2×2N splitter as shown in FIG. 2. The 2×2N splitter comprises 3 dB coupler 9 independent of its wavelength with a waveguide circuit same as a Mach-Zender inteferometer type optical circuit 17, and a waveguide branch circuit 10 having paralleled two 1×2N splitter in series with said coupler 9. This 2×2N splitter as shown in FIG. 2 achieves a function of light power splitter with a wave band required for DWDM.
For example, according to the 2×2N splitter shown in FIG. 2, it is possible to deliver equally signal as separate outputs 101, 102, . . . 102N of waveguide branch circuit 10 to receive optical signals each having a band width of 80–100 nm around 1550 nm from a terminal 9a of the 3 dB coupler 9 independent of its wavelength. In case signals are received at 9b, signals are output at terminals 101, 102, . . . 102N as a same manner. In ordinary communication system, taking into account of security, either one of terminal 9a or 9b is reserved as a supplement.
However, because the 2×2N splitter shown in FIG. 2 includes the waveguides made from a polymer, its optical characteristics are apt to vary in the presence of changes of the ambient temperature, and thus the temperature range under which it can normally operate is rather limited. Further a polymer has a birefringence and the characteristics are apt to vary in the presence of the direction of light polarization used. Thus the loss characteristics dependent of polarization (PDL) is rather worse. However, said waveguide branch circuit 10 of the rear part of the 2×2N splitter shown in FIG. 2 made from a polymer waveguides are unaffected by the ambient temperature and the change of the polarization. (Bao-Xue Chen et al, “Optical Coupler”, U.S. Pat. No. 5,757,995). Therefore the 3 dB coupler 9 independent of wavelength of the front part of the 2×2N splitter shown in FIG. 2 should be carefully designed. The temperature coefficient of the refractive index of a polymer material used for the construction of the waveguides is ten or more times as high as that of quartz and its birefringence (a double refraction) is about 0.0082, as that of quartz is about Zero. Therefore, if the ambient temperature changes, the polymeric waveguides 9, 10 and cladding layers covering those waveguides will undergo a great change in their refractive indices; the parameters of the optical circuit including those waveguides and cladding layers will also shift from the designed ranges, and the performance of the optical circuit will depart from the designed level.
Analyzing a Mach-Zender inteferometer type optical circuit 17 shown in FIG. 9, as the same structure as the 3 dB coupler 9 independent of its wavelengh, When a refraction of said waveguide core 13 and 14, and the cladding around said waveguide core change remarkably due to the alteration of the temperature, degraded performance as a result of this alteration of the ambient temperature is mainly ascribed to the following two reasons; The first reason is; a combination factor designed is modified due to a change of a complete combination length of said two directional coupler 15 and 16. The second reason is; a power distribution ratio is modified in the presence of a phase difference which is produced as a result of properly chosen path length difference between two waveguide core 13 and 14. Further a equivalent refraction of a waveguide is modified due to a effect of a birefringence (a double refraction) in the presence of a change of a polarization plane of a input signal. Therefore, because a complete combination length of two directional couplers 15 and 16, and a phase difference which is produced as a result of properly chosen path length difference between the two waveguide cores 13 and 14 will change, a power distribution ratio will shift from the designed ranges.
For solving the problem on a dependence of temperature, it is necessary to redesign the overall structure. Similar problems above are also observed in certain types of quartz optical waveguide circuits. The refractive index of a quartz material has a positive temperature coefficient whose absolute value is smaller than that of a corresponding polymeric material. Therefore, a known method for preparing an optical waveguide circuit from quartz consists of covering a quartz waveguide core with a polymeric coat whose refractive index has a negative temperature coefficient sufficiently large to cancel the positivity of the temperature coefficient of the quartz waveguide core. However, generally a polymeric material has a refractive index whose temperature coefficient has too large a negative value to cancel the positivity of the temperature coefficient of a quartz material. Naturally, this method can not be applied for the polymeric optical waveguide circuit here concerned.
A known method for compensating for the thermal characteristics of a polymeric waveguide core is to employ a substrate made from a polymer having a high thermal expansion. To put it more specifically, this method consists of employing a polymeric substrate which has a thermal expansion sufficiently high to cancel the negative temperature coefficient of the refractive index of a polymeric waveguide core. However, a substrate made from quartz or silicon generally has a low thermal expansion, and thus as far as based on this method, it will not be possible to integrate optical waveguide circuits on a silicon substrate as in the conventional electronic technology where semiconductor devices are integrated on a silicon substrate.